Fault-tolerant IS-IS routing system, and a corresponding method

ABSTRACT

The invention provides a high availability control method for a router in an autonomous system, the router being in communication with other routers using an IS-IS protocol via interfaces and presenting both an active IS-IS protocol engine and a standby IS-IS protocol engine. The method comprises steps of storing data concerning adjacency, the state of links, and interfaces in a memory of the active protocol engine, updating the data stored in a memory of the standby protocol engine, and activating the standby protocol engine using the updated data. The invention also provides a router implementing the method. The invention serves to increase router availability in the event of a failure or of maintenance actions.

[0001] The present invention relates to IS-IS protocol routing inautonomous systems, and it relates more particularly to internetprotocol (IP) routers.

BACKGROUND OF THE INVENTION

[0002] Routers, also known as intermediate systems “IS”, are known, forexample the Alcatel 7670 RSP router, which present operatingdiscontinuities due to periods of unavailability. Periods ofunavailability are due firstly to planned operations, such as routermaintenance, and secondly to unexpected events, such as router failures.When other routers detect that a router has stopped or failed, theirdata concerning routing via the unavailable router is made invalid. Whenthe router is put back into operation, all of that routing data is lostand needs to be exchanged anew with the other routers. This gives risefirstly to an increase in traffic on the network. It also gives rise toproblems of accessibility on the network. Periods of routerunavailability thus inconvenience users of the network.

[0003] A project for extending the IS-IS protocol has been proposed bythe internet engineering task force (IETF) under the reference “Restartsignaling for IS-IS” in order to enable programmed restarting of arouter in a manner that reduces those drawbacks. However thecorresponding protocol is neither available nor even tested. Thatproject makes no provision for unplanned restarts. That project does notappear to be adequate for providing a sufficient increase in routeravailability. That project also requires changes to the standardsconcerning the IS-IS protocol. There is no prospect in the short term ofchanging the standards of the IS-IS routing protocol since that wouldrequire present routers to be updated or replaced.

OBJECTS AND SUMMARY OF THE INVENTION

[0004] There therefore exists a need for a router which solves one ormore of the drawbacks in the state of the art.

[0005] The invention thus proposes a method of controlling a router inan autonomous system, the router being in communication with otherrouters using an IS-IS protocol via interfaces and presenting: an activeIS-IS protocol engine; and a standby IS-IS protocol engine; the methodcomprising the steps of: the router communicating with other routers viathe active protocol engine; storing the following in a memory of theactive protocol engine: data concerning the adjacency of the otherrouters; data concerning the state of links with the other routers; anddata concerning the interfaces; updating the data stored in a memory ofthe standby protocol engine on the basis of the data in the activerouter concerning adjacency, the state of the links, and the interfaces;and activating the standby protocol engine with the updated data, byusing the IS-IS protocol with the other routers.

[0006] In a variant, all of the data is updated at the request of thestandby protocol engine.

[0007] In another variant, the method further comprises a step ofdetecting a modification of the data stored in a memory of the activeprotocol engine, with updating being performed whenever a modificationof said data is detected.

[0008] In another variant, the detected modification is selected fromthe group constituted by: adjacency activation; adjacency deactivation;adjacency data being modified, possibly after receiving a presencedeclaration packet; modifications, deletions, and creations relating tothe states of links and of interfaces.

[0009] In yet another variant, the step of activating the standbyprotocol engine includes a step of validating the preserved adjacencyand link state data without modifying the IS-IS protocol.

[0010] Provision can also be made for the standby protocol engine toperform a shortest path search on the basis of the updated data.

[0011] In a variant, activation of the standby protocol engine includesa step of verifying the validity of its adjacency data.

[0012] In another variant, validity verification comprises: sending anIIH PDU data packet from the standby protocol engine to an adjacentrouter, the packet containing a request that the adjacent router send aCSNP data packet; and modifying the adjacency data as a function of thenature of the response from the adjacent router.

[0013] The invention also provides a communication method comprising thefollowing steps: sending an IIH PDU data packet from a first router toan adjacent second router, the packet including a parameter at apredetermined location; sending a CSNP data packet from the secondrouter to the first router as a function of the value of said parameterof the IIH PDU data packet.

[0014] The invention also provides an IS-IS communication protocol inwhich an IIH PDU data packet contains a request for a CSNP data packetto be sent.

[0015] The invention also provides a router presenting a plurality ofinterfaces via which it is capable of communicating with other routersusing an IS-IS protocol, the router comprising: an active IS-IS protocolengine; a standby IS-IS protocol engine; a communications channelbetween the protocol engines; a least one data storage memory incommunication with the active protocol engine; and at least one datastorage memory in communication with the standby protocol engine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Other characteristics and advantages of the invention appear onreading the following description of embodiments of the invention givenby way of example and with reference to the drawings, in which:

[0017]FIG. 1 is a diagram of a router in accordance with the invention;and

[0018]FIG. 2 is a diagram of a communications network in which one ormore FIG. 1 routers are used.

MORE DETAILED DESCRIPTION

[0019] The invention thus proposes a method in which the databases of anactive IS-IS protocol engine are used to update the databases of anIS-IS protocol engine on standby. The active protocol engine uses adetermined IS-IS routing protocol for communication with other routersin an autonomous system or AS. The term “autonomous system” or “AS” isused to designate an autonomously controlled administrative entity. Whenthe active protocol engine becomes unavailable, the standby protocolengine becomes active and communicates with other routers, using theupdated databases and the routing protocol as used by the activeprotocol engine.

[0020]FIG. 1 is a diagram showing the structure of a router 1 of theinvention. This router 1 comprises an active protocol engine 2. Therouter 1 also comprises a standby protocol engine 3 designed to takeover from the active protocol engine 2 in the event of it failing. TheseIS-IS protocol engines are defined in the request for comment (RFC) No.1142 standard of February 1990 and in the international standardsorganization (ISO) standard 10589 version 2. The protocol engines 2 and3 are connected together by a connection 6 which forms a communicationschannel between them. A switch 4 enables the active protocol engine 2 orthe standby protocol engine 3 to be put into communication selectivelywith the inlet/outlet interfaces 5 of the router. Although thecommunications protocol engines 2 and 3 in the example shown areconstituted by different hardware components, such as different cardsconnected together by the connection 6, it is naturally also possible toprovide for the protocol engines to be formed by executable programsrunning in parallel, preferably on distinct hardware elements that arecapable of communicating with each other.

[0021] The active protocol engine 2 includes or is connected to astorage memory, e.g. a cache memory. In similar manner, the standbyprotocol engine 3 includes or is connected to a storage memory, whichcan likewise be a cache memory.

[0022] The memories of the protocol engines serve in particular to storeadjacency databases 21 and 31, databases 22 and 32 concerning the stateof links, and databases 23 and 33 concerning the state of theinterfaces.

[0023] In general manner, the content of the databases of a protocolengine is specified in the standards RFC 1142 and 1195.

[0024] An adjacent database (ADB) contains in particular descriptions ofother IS-IS protocol engine routers to which the router 1 is connected.

[0025] A link state database (LSP DB) contains data concerning the stateof links throughout the networks and the routers of the autonomoussystem or AS.

[0026] By way of example, an interface state database contains flagsconcerning the state of interfaces, information about interfaceparameters, or indeed about the presence of a router DIS connected to aninterface.

[0027] It is also possible to provide each protocol engine with adatabase containing timers.

[0028] The method of operation of the router may be as follows.

[0029] Initially, a step of starting the standby protocol engine 3 canbe provided. This starting of the standby protocol engine is performed,for example, when the router is switched on. During this starting step,the functions of sending “hello” packets or IIH by the standby protocolengine 3 are inhibited, for example. Hello or IIH packets are packetsfor signaling presence that are generally sent at regular intervals by aprotocol engine over the network to indicate that the router is presentand to give information concerning it. This inhibition may beimplemented, for example, by blocking the corresponding timers in thestandby protocol engine.

[0030] In parallel, the active protocol engine 2 communicates by usingIS-IS protocol with other routers of the autonomous system AS. Whilecommunicating with other routers, the active protocol engine recoversdata concerning interfaces, adjacent routers, and the state of links.The active protocol engine receives in particular IS-IS data coming fromother routers in the form of hello packets, in the form of LSP packets,or link state packets, in the form of CSNP packets, i.e. a list of theLSPs known to a router, or in PSNP form. The active protocol enginestores this data in its memory in the databases 23, 21, and 22respectively.

[0031] Certain databases of the active protocol engine 2, and inparticular the databases containing interface data, link state data, andadjacency data, are subsequently transmitted in full to the standbyprotocol engine 3. Naturally, the databases can be sent in full atintervals that are spaced apart in time in the form of packets so as toavoid interfering with the operation of the active protocol engine 2.Provision can be made for the standby protocol engine 3 to deliveracknowledgments of receipt to the active protocol engine 2. The activeprotocol engine 2 then knows that the data in the standby protocolengine 3 has been updated.

[0032] This transmission may be performed either on the initiative ofthe active protocol engine 2, or at the prior request of the standbyprotocol engine. Provision can thus be made for the standby protocolengine 3 to request resynchronization, with the active protocol engine 2responding thereto by sending databases. Such a request is sent, forexample, at the end of the process of starting the standby protocolengine 3.

[0033] When starting a router, a local memory zone is created, forexample, in the standby protocol engine to store databases concerningadjacencies, link states, and interfaces. The memory enables data to becreated, modified, and deleted.

[0034] The data received is then stored in the databases of the standbyprotocol engine 3. The databases containing interface data, link states,and adjacency data in the standby protocol engine 3 are then up to date,thus enabling the standby engine to be activated should that benecessary in the event of the active protocol engine 2 failing. Thestandby protocol engine 3 may also prepare to receive and processsubsequent updates.

[0035] In a variant, the active protocol engine subsequently sends itsdatabases to the standby protocol engine 3. It is preferable to updatethe data in the standby protocol engine 3 incrementally, i.e. by sendingonly data that has changed since the most recent update to the standbyprotocol engine 3.

[0036] Provision can thus be made to detect any modification to the datain the memory of the active protocol engine. The data in the standbyprotocol engine 3 can then be updated whenever a modification isdetected. In particular, updating can be performed if one of thefollowing modifications is detected: activating adjacency; deactivatingadjacency; modifications to adjacency data optionally due to receiving apacket declaring presence; or modifications, deletions, and creationsrelating to the state of links and of interfaces. This variant makes itpossible to send incremental data updates from the active protocolengine 2 to the standby protocol engine. This can reduce the quantity ofdata that is exchanged between the protocol engines. This avoids theactive protocol engine 2 being excessively occupied in sending data tothe detriment of performing routing tasks. Such updating guarantees thatthe database of the standby protocol engine 3 benefits from beingrecently up to date in the event of the active protocol engine 2failing.

[0037] Provision can be made for the standby protocol engine 3 toimplement a process for finding shortest paths or SPF on the basis ofits own databases updated prior to its activation.

[0038] There follows a description of an example of activating thestandby protocol engine.

[0039] Following one or more updates, the standby protocol engine 3 hasone of the latest images of adjacency data, interface data, and linkstate data from the active protocol engine 2 in its own memory. Thestandby protocol engine is thus ready to take over from the activeprotocol engine in the event of the active protocol engine ceasing tooperate. The standby protocol engine can then operate as the activeprotocol engine using the IS-IS protocol initially used by the activeprotocol engine. Activating the standby protocol engine then correspondsto warm starting the router with operational routing information.

[0040] When the standby protocol engine switches to its active state,its configuration may be reinitialized. The standby protocol engine maycreate preserved interfaces, adjacencies, and link states. Preservedinterfaces, adjacencies, and link states are adjacencies whose state isconsidered as being active or UP after the latest update of the standbyprotocol engine.

[0041] During its activation, the inhibits on the standby protocolengine are eliminated. The times for sending data such as LSPs and hellopackets are activated. The standby protocol engine then sends LSPs tothe other routers. The standby protocol engine verifies the validity ofits own databases. In particular, it verifies the validity of its ownadjacency database in a so-called “2-way check”. It also verifies thevalidity of its own LSP database.

[0042] For the step of restarting an interface, the following sequenceof events can be provided:

[0043] Initially, a search is made for the UP adjacencies connected tothe given interface. These adjacencies are then classified as a functionof the role they perform in the network. For example, an adjacent routerserving only to perform internal routing within an area of theautonomous system or AS is referred to as being of “level 1”. Anadjacent router having a routing function in the backbone of theautonomous system AS is referred to as being of “level 2”. The way inwhich interfaces are restarted varies depending on interface type.

[0044] In the example of FIG. 2, the cross-ruled portion corresponds toa first area 7. The spotted portion corresponds to a second area 8. Theportion having a white background corresponds to the backbone of theautonomous system AS. Routers lying within the solid outlines 10 and 11are level 1 routers. Routers lying within the dashed outlines are level2 routers.

[0045] Two types of interface are distinguished: an interface providingconnection with a local area network (LAN) which may be a broadcast or anon-broadcast space multiple-access network, e.g. the interfaceproviding a connection between routers 13 and 14; and a point-to-point(PTP) connection interface, e.g. the connection interface betweenrouters 12 and 13.

[0046] LAN Interface

[0047] If a level 1 or 2 adjacency is UP on the interface, a data packet(PDU) of the IS-IS hello type (IIH) for a LAN network is prepared. Thelist of all of the same-level adjacencies that are UP is included forthis interface in the appropriate TLV field of this IIH PDU data packet.The connection level local address or MAC address is also addedconcerning each of these adjacencies in the IIH PDU packet. Thereafterthe IIH PDU packet is sent over the interface. The adjacencies processedin this way are marked as being in the “2-way check” state in theadjacency database of the router.

[0048] PTP Interface

[0049] If an adjacency has been found, and regardless of its role level,the adjacency is marked as being in the “2-way check” state in theadjacency database of the router. An IIH PDU packet is prepared and thensent over the interface. The MAC address of the adjacency is included inthis IIH PDU packet. If the “3-way handshake” protocol extension isavailable, a “point-to-point adjacency state” option may be added to theIIH PDU packet. This option makes it possible to recover the states seenby adjacent routers in the TLV field of the IIH PDU packet. The“adjacency state” value of this option must then mention an UP state. Ifother fields of the “adjacency state” option are supported, e.g. theextended MAC address field, they are also used in the IIH PDU packet assent.

[0050] After an interface has restarted, all of the correspondingadjacencies which were in the UP state during the failure are placed inthe “2-way check” state. The “2-way check” state is not visible to otherrouters. In addition, for each interface possessing at least oneadjacency in the “2-way check” state, a timer is initialized forrestarting the interface. The value of this timer is equal to the valueof the “holding timer” of the interface in question.

[0051] When an adjacency is in the “2-way check” state, the followingsequence of events is provided, depending on the type of adjacency.

[0052] PTP Adjacency

[0053] If a PDU packet of the CSNP link state group type or the LSP linkstate type is received, the adjacency is marked as being in the activeor UP state, and the data packet is processed in the manner specified inthe IS-IS protocol standard.

[0054] If an IIH PDU packet is received, several circumstances can bedistinguished:

[0055] The IIH PDU Packet Presents an Adjacency State Option Field:

[0056] If the adjacency state option field is UP (and possibly if otheroption state fields are included in the PDU and all corresponding to theadjacency state), the adjacency is marked as being in the synchronizedstate SYNC in the database.

[0057] If the adjacency state option field is not UP (or possibly ifother state option fields are included in the PDU and do not allcorrespond to the adjacency state), then the adjacency is marked asbeing in the initialization state INITIALIZE in the database.

[0058] The IIH PDU Packet does not have the Adjacency State OptionField:

[0059] If the source identification and the local circuit fields of thereceived PDU packet correspond to adjacency, then the adjacency ismarked as being in the synchronized state SYNC. Otherwise the adjacencyis marked as being in the initialization state INITIATE in the database.

[0060] LAN Adjacency

[0061] If a PDU packet of the CSNP type is received, if this packetindicts that the router 1 is part of the adjacencies of the sendingrouter, and if the sending router is marked as being in the “2-waycheck” state in the database of router 1, then the adjacency of thesending router is marked as being in the UP state. The received PDU datapacket is then processed in the manner specified in the IS-IS protocolstandard.

[0062] If an IIH PDU packet is received, the list of MAC addressescontained in the adjacent routers option field of the PDU packet isexamined. Thereafter a search is made to see whether the MAC address ofrouter 1 is present in the list of MAC addresses that have beenexamined.

[0063] The LAN-ID field of the received PDU packet is also examined todetermine whether router 1 serves as a designated router or DIS of level1 or level 2.

[0064] Thereafter, a search is made for an adjacency whose MAC addresscorresponds to the MAC address of the sender of the PDU packet.

[0065] If this adjacency is found and if it is marked as being in the“2-way check” state, then:

[0066] if the MAC address of router 1 is present in the list of MACaddresses in the adjacent routers field of the received PDU packet, theadjacency state is marked as being in SYNC; or

[0067] if the MAC address of router 1 is not in the list of the adjacentrouters field of the received PDU packet, then this adjacency isdeleted.

[0068] If no adjacency is found or if it is not marked as being in the“2-way check” state, the data packet is processed as being a normal datapacket coming from said adjacency.

[0069] When an adjacency is in the SYNC state, the router 1 is informedthat both-way communication with the corresponding adjacent router isoperational. However router 1 does not know whether the link statespreviously transmitted by the adjacent router and stored by the standbyprotocol engine are up to date. There therefore follows a detaileddescription of a step of synchronizing adjacencies concerning linkstates.

[0070] Synchronization for a PTP Adjacency

[0071] If a PDU data packet of CSNP type is received and if theadjacency corresponding to the sending router is marked as being in theSYNC state, then the adjacency is marked as being UP. The received PDUpacket is then processed in the manner defined by the IS-IS protocolstandard.

[0072] If a PDU packet of a type other than CSNP is received and if theadjacency corresponding to the sending router is marked as being in theSYNC state, the adjacency is then marked as being UP. The received PDUpacket is then processed in the manner defined in the IS-IS protocolstandard.

[0073] Synchronization for a LAN Adjacency

[0074] If a PDU data packet of CSNP type is received, if the adjacencycorresponding to the sending router is marked as being in the SYNCstate, and if router 1 is not the DIS, then the adjacency is marked asbeing in the UP state. The PDU packet is then processed in the mannerdefined in the IS-IS protocol standard.

[0075] If a PDU data packet of a type other than CSNP is received froman adjacent sending router, the adjacency remains marked as being in theSYNC state. The PDU packet is then processed in the manner defined inthe IS-IS protocol standard.

[0076] There follows a description of the behavior of the activatedprotocol engine 3 that was initially on standby, in the event of aninterface restarting timer timing out.

[0077] Timeout for a PTP Interface

[0078] A search is initially made in the adjacency corresponding to theinterface that has timed out.

[0079] If the adjacency is marked as being in the “2-way check” state,it is deduced that the router corresponding the adjacency has becomeunavailable during the period between failure of the active protocolengine and activation of the standby protocol engine. The adjacencycorresponding to this router is then deleted.

[0080] If the adjacency is marked as being in the SYNC state, then thestate of this adjacency is not changed. A PDU packet of the CSNP type issent. This CNSP PDU packet contains an LSP corresponding to theadjacency and having a checksum that has been deliberately altered so asto be invalid (i.e. so as to have a value that does not correspond tothe value stored in the link state database of the activated engine 3that was initially on standby). Thus, the adjacent router in questionresponds to the CSNP PDU packet by sending an LSP update to router 1 inorder to replace the deliberately invalid LSP, together with other LSPsthat might not have been recorded prior to activation of the standbyprotocol engine.

[0081] If the adjacency remains in the SYNC state, then the timer isreinitialized. Provision can be made to count the number of timeouts.The adjacency is then deleted if the number of timeouts reaches adetermined threshold.

[0082] For other markings of the state of adjacency, timeout is ignored.

[0083] By repeating the process for the various interfaces, by the endof activation, the database of the activated protocol engine 3 that wasinitially on standby contains only UP adjacency states. Thus, only thoseadjacencies that are active remain marked in the database.

[0084] Timeout for a LAN Interface

[0085] Initially, a search is made in all of the adjacenciescorresponding to the interface that has timed out.

[0086] If an adjacency is marked as being in the “2-way check” state, itis deduced that the router corresponding to the adjacency becameunavailable in the period between the active protocol engine failing andthe standby protocol engine being activated. The adjacency correspondingto this router is therefore deleted.

[0087] If an adjacency is marked as being in the SYNC state, and if therouter 1 is the DIS router, then the state of this adjacency is markedas being UP. If a CSNP type PDU packet has not already been sent forthis interface, such a packet is sent now. This CSNP PDU packet includesall of the link states of level identical to that of the adjacency inits database.

[0088] If an adjacency is marked as being in the SYNC state, and ifrouter 1 is not the DIS router, the state marking of this adjacency isnot modified.

[0089] If at least one adjacency of the interface remains in the SYNCstate, the timer is reinitialized. Provision can be made to count anumber of timeouts. The adjacencies of the interface are then deleted ifthe number of timeouts reaches a determined threshold.

[0090] For other markings of the adjacency state, timeout is ignored.

[0091] By repeating the process for the various interfaces, by the endof activation the database of the activated protocol engine 3 that waspreviously on standby contains only UP adjacency states. Thus, onlythose adjacencies which are active remain marked in the database.

[0092] There follows a description of a possible extension to the IS-ISprotocol for facilitating the step of activating the standby protocolengine. It is proposed to integrate a new option requesting that a CSNPbe sent in IIH (IS-IS hello) PDU data packets. A sender routercompatible with the extension could then, for example, integrate thefollowing fields in an IIH PDU:

[0093] Field “CSNP request type”: a predetermined field value can beprovided for which the sending of a CSNP is requested of the destinationas soon as possible. The destination router must be marked as being inthe UP state in the adjacency database and the sending router must notbe the DIS router in the case of a LAN type subnetwork.

[0094] If a destination router compatible with the extension to theIS-IS protocol receives the IIH PDU packet, it performs the followingprocessing:

[0095] If the packet comes from a LAN type subnetwork for which thedestination router serves as a DIS, and if the adjacency correspondingto the sending router is in the UP state, a CSNP packet is sent as soonas possible by the destination.

[0096] If the IIH PDU packet comes from a PTP subnetwork, and if theadjacency corresponding to the sending router is in the UP state, a CSNPpacket is sent as soon as possible by the destination.

[0097] Else, the option field is ignored and the destination router doesnot send a CSNP packet. As explained above, the marking of the adjacencystate can be deleted when no data packet has been returned from theadjacency after repeated timeouts.

[0098] If a destination router that is not compatible with the protocolextension receives the IIH PDU packet, the option field is ignored.

[0099] In all cases, the remainder of the IIH PDU packet is processesnormally by the destination router.

[0100] Overall, the marking of the adjacency state is modified as afunction of the nature of the response received from the adjacentrouter. Naturally, it is considered that the absence of any responsefrom the adjacent router constitutes a response of a particular kind.

[0101] The method described makes it possible to reduce routerunavailability and to make the failure of a protocol engine invisible toother routers. Activation of the standby protocol engine is thus notdetected by the other routers.

[0102] The present embodiments and examples should be considered asbeing given by way of non-restrictive illustration and the invention isnot limited to the details provided herein, but may be modified whileremaining within the scope of the accompanying claims.

What is claimed is: 1/ A method of controlling a router in an autonomoussystem, the router being in communication with other routers using anIS-IS protocol via interfaces and presenting: an active IS-IS protocolengine; and a standby IS-IS protocol engine; the method comprising thesteps of: the router communicating with other routers via the activeprotocol engine; storing the following in a memory of the activeprotocol engine: data concerning the adjacency of the other routers;data concerning the state of links with the other routers; and dataconcerning the interfaces; updating the data stored in a memory of thestandby protocol engine on the basis of the data in the active routerconcerning the adjacency, the state of the links, and the interfaces;and activating the standby protocol engine with the updated data, byusing the IS-IS protocol with the other routers. 2/ The method of claim1, wherein all of the data is updated at the request of the standbyprotocol engine. 3/ The method of claim 1, further comprising a step ofdetecting a modification of the data stored in a memory of the activeprotocol engine, with updating being performed whenever a modificationof said data is detected. 4/ The method of claim 3, wherein the detectedmodification is selected from the group constituted by: adjacencyactivation; adjacency deactivation; adjacency data being modified,possibly after receiving a presence declaration packet; modifications,deletions, and creations relating to the states of links and ofinterfaces. 5/ The method of claim 1, wherein the step of activating thestandby protocol engine includes a step of validating the preservedadjacency and link state data without modifying the IS-IS protocol. 6/The method of claim 1, wherein, prior to being activated, the standbyprotocol engine performs a shortest path search on the basis of theupdated data. 7/ The method of claim 1, wherein activation of thestandby protocol engine includes a step of verifying the validity of itsadjacency data. 8/ The method of claim 7, wherein validity verificationcomprises: sending an IIH PDU data packet from the standby protocolengine to an adjacent router, the packet containing a request that theadjacent router send a CSNP data packet; and modifying the adjacencydata as a function of the nature of the response from the adjacentrouter. 9/ A communication method comprising the following steps:sending an IIH PDU data packet from a first router to an adjacent secondrouter, the packet including a parameter at a predetermined location;sending a CSNP data packet from the second router to the first router asa function of the value of said parameter of the IIH PDU data packet.10/ An IS-IS communication protocol in which an IIH PDU data packetcontains a request for a CSNP data packet to be sent. 11/ A routerpresenting a plurality of interfaces via which it is capable ofcommunicating with other routers using an IS-IS protocol, the routercomprising: an active IS-IS protocol engine; a standby IS-IS protocolengine; a communications channel between the protocol engines; a leastone data storage memory in communication with the active protocolengine; and at least one data storage memory in communication with thestandby protocol engine.